3-dehydroquinate dehydratase (EC 4.2.1.10), or dehydroquinase, catalyzes the conversion of 3-dehydroquinate into 3-dehydroshikimate. It is the third step in the shikimate pathway for the biosynthesis of aromatic amino acids from chorismate. Two classes of dehydroquinases exist, known as types I and II. The best studied type I enzyme is from Escherichia coli (gene aroD) and related bacteria where it is a homodimeric protein of a chain of about 250 residues. In fungi, dehydroquinase is part of a multifunctional enzyme which catalyzes five consecutive steps in the shikimate pathway. In aroD, it has been shown [1] that a histidine is involved in the catalytic mechanism; we used the region around this residue as a signature pattern.